Research

Each year, approximately 500,000 patients, including those with rheumatoid arthritis and osteoarthritis, undergo total hip, knee and other joint replacement surgery in the United States.  The clinical performance of implanted devices can be followed and studied.  In addition, a fraction of these implants are removed each year for reasons such as infection, osteolysis, or mechanical failure of the components.  The removed or revised components can be preserved for analysis.  They provide a rich resource with which to examine the influence of surgical, patient, design, and material factors on the long-term performance of these devices.

The overall objective of the Center for the Evaluation of Implant Performance is to correlate evidence of the mechanical behavior of implanted and retrieved devices with clinical, material, design, and manufacturing variables.  This objective is achieved, in part, by collaborating as the key secondary site in a multi-center Hip and Knee Retrieval Repository Program based at Drexel University under the direction of Steven Kurtz, Ph.D. 

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Recent Publication Highlights

Du, Jerry, Flanagan, Christopher, Bensusan, Jay, Knusel, Konrad, Akkus, Ozan, Rimnac, Clare. Raman Biomarkers Are Associated with Cyclic Fatigue Life of Human Allograft Cortical Bone. J Bone Joint Surg Am. 2019;101(17):e85. 

Background: Structural bone allografts are an established treatment method for long-bone structural defects resulting from such conditions as traumatic injury and sarcoma. The functional lifetime of structural allografts depends on resistance to cyclic loading (cyclic fatigue life), which can lead to fracture at stress levels well below the yield strength. Raman spectroscopy biomarkers can be used to non-destructively assess the 3 primary components of bone (collagen, mineral, and water), and may aid in optimizing allograft selection to decrease fatigue fracture risk. We studied the association of Raman biomarkers with the cyclic fatigue life of human allograft cortical bone.
Methods: Twenty-one cortical bone specimens were machined from the femoral diaphyses of 4 human donors (a 63-year old man, a 61-year-old man, a 51-year-old woman, and a 48-year-old woman) obtained from the Musculoskeletal Transplant Foundation. Six Raman biomarkers were analyzed: collagen disorganization, mineral maturation, matrix mineralization, and 3 water compartments. The specimens underwent cyclic fatigue testing under fully reversed conditions (35 and 45 MPa), during which they were tested to fracture or to 30 million cycles (“runout”), simulating 15 years of moderate activity. A tobit censored linear regression model for cyclic fatigue life was created.
Results: The multivariate model explained 60% of the variance in the cyclic fatigue life (R2=0.604,p<0.001). Increases in Raman biomarkers for disordered collagen (coefficient: 22.74·107, p < 0.001) and for loosely collagen-bound water compartments (coefficient: 22.11·108, p < 0.001) were associated with a decreased cyclic fatigue life. Increases in Raman biomarkers for mineral maturation (coefficient: 3.50·108, p < 0.001), matrix mineralization (coefficient: 2.32·106, p < 0.001), tightly collagen-bound water (coefficient: 1.19·108, p < 0.001), and mineral-bound water (coefficient: 3.27·107, p < 0.001) were associated with an increased cyclic fatigue life. Collagen disorder accounted for 44% of the variance in the cyclic fatigue life, mineral maturation accounted for 6%, and all bound water compartments accounted for 3%.
Conclusions: Increasing baseline collagen disorder was associated with a decreased cyclic fatigue life and had the strongest correlation with the cyclic fatigue life of human cortical donor bone. This model should be prospectively validated.
Clinical Relevance: Raman analysis is a promising tool for the non-destructive evaluation of structural bone allograft quality for load-bearing applications.

 

Electrochemical Impedance Spectroscopy As a Method to Distinguish Corrosion Severity and Damage Modes in Retrieved Femoral Heads. GB Higgs, CM Rimnac, WM Mihalko, JL Gilbert, SM Kurtz. Orthopaedic Proceedings 101 (SUPP_5), 142-142.

Introduction:  Corrosion at modular junctions of total hip replacements has been identified as a potential threat to implant longevity, resulting in efforts to determine appropriate countermeasures. Visual scoring and volumetric material loss measurements have been useful tools to elucidate various clinical and design factors associated with corrosion damage. However, corrosion involves electron exchange that results in chemical changes to biomedical alloys, and electrochemical assessment may therefore be a more appropriate approach to understand the phenomenon. The purpose of this pilot study was to electrochemically distinguish the severity of corrosion in retrieved femoral heads. A secondary goal was to identify the potential of electrochemical impedance spectroscopy (EIS) as a method to identify different forms of corrosion damage.

Methods:  Twenty femoral heads were identified from a larger study of total hip replacements, obtained as part of an ongoing multi- center IRB-approved retrieval program. Using a previously established 4-point scoring method, components were binned by taper damage: 10 components were identified as having severe damage, 7 with moderate damage and 2 with mild damage. One (1) unimplanted control was included to represent minimal corrosion damage. All components were then characterized using electrochemical impedance spectroscopy under the frequency domain: a 10 mV sinusoidal voltage, ranging from 20 kHz to 2 mHz, was applied to the taper of a femoral head (working electrode) filled with a 1M solution of PBS, a platinum counter electrode and a chlorided silver reference electrode. Absolute impedance at 2 mHz (|Z0.002|), and max phase angle (θ) were assessed relative to taper damage severity. After least-squares fitting of the EIS data to a Randles circuit with a constant phase element, circuit elements: polarization resistance (Rp), CPE-capacitance, and CPE-exponent were also evaluated. The seven (7) most severely corroded components were further examined with scanning electron microscopy to identify corrosion modes. For all statistical analyses, significance was determined at alpha=0.05.

Results: Taper damage was strongly correlated with both |Z0.002| (ρ = −0.857, p<0.001) and CPE-capacitance (ρ=0.913, p<0.001). Taper damage was moderately associated with max phase angle (ρ= −0.483, p=0.031), CPE-exponent (ρ= −0.653, p=0.002) and Rp (ρ=0.556, p=0.011). Log-log plots of the strongest predictors of taper damage (|Z0.002| and CPE- capacitance) identified some clustering among severely corroded components. SEM analysis identified evidence of grain/phase boundary corrosion on four components, all with log CPE-capacitance ≥ −4.4.

Discussion:  The results of this pilot study highlight that electrochemical impedance spectroscopy is useful in determining corrosion severity in retrieved femoral heads, and may also identify intergranular corrosion attack. For an undamaged taper, the self- passivating behavior of CoCrMo creates a surface that opposes charge transfer, but greater corrosion appears to compromise this barrier. The observed trend of low impedance but high capacitance for severely corroded components with intergranular corrosion may signal charge storage at the boundaries of individual grains. Additional work is underway to characterize this

 

Crack initiation from a clinically relevant notch in a highly-crosslinked UHMWPE subjected to static and cyclic loading. A Sirimamilla, CM Rimnac. Journal of the mechanical behavior of biomedical materials 91, 366-372.

Crosslinked Ultra High Molecular Weight Polyethylene (UHMWPE), which is used as a bearing material in total joint replacement components, is subjected to static and cyclic loads in vivo. Resistance to crack initiation from a notch as a function of static and cyclic loads is not well understood for crosslinked UHMWPE. This study estimated the resistance of crosslinked UHMWPE (crosslinked with 100 kGy gamma radiation and remelted to extinguish free radicals) to crack initiation for a clinically relevant notch under both static and cyclic loading conditions. For cyclic loading, four frequencies were applied with a sine waveform and two frequencies were applied with a square waveform to independently estimate the effect of frequency and rate of loading on crack initiation. Crack initiation time and cycles to crack initiation were determined. Crack initiation time for fatigue loading conditions was substantially lower compared to static loading conditions. Crack initiation time decreased with an increase in test frequency. A square wave resulted in shorter crack initiation time compared to a sine wave. The results suggest that crosslinked UHMWPE is more resistant to crack initiation from a notch under static loading conditions compared to fatigue loading conditions.